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1 g 2g_3g_4g_tutorial

2.
Cellular Network Basics
∗ There are many types of cellular services; before delving into
details, focus on basics (helps navigate the “acronym soup”)
∗ Cellular network/telephony is a radio-based technology; radio
waves are electromagnetic waves that antennas propagate
∗ Most signals are in the 850 MHz, 900 MHz, 1800 MHz, and 1900
MHz frequency bands
Cell phones operate in this frequency
range (note the logarithmic scale)

3.
∗ Base stations transmit to and receive from mobiles at the
assigned spectrum
∗ Multiple base stations use the same spectrum (spectral reuse)
∗ The service area of each base station is called a cell
∗ Each mobile terminal is typically served by the ‘closest’ base
stations
∗ Handoff when terminals move
Cellular Network

6.
∗ The base stations need to serve many mobile
terminals at the same time (both downlink and
uplink)
∗ All mobiles in the cell need to transmit to the base
station
∗ Interference among different senders and receivers
∗ So we need multiple access scheme
The Multiple Access Problem

8.
∗ Each mobile is assigned a separate frequency channel for the
duration of the call
∗ Sufficient guard band is required to prevent adjacent channel
interference
∗ Usually, mobile terminals will have one downlink frequency band
and one uplink frequency band
∗ Different cellular network protocols use different frequencies
∗ Frequency is a precious and scare resource. We are running out of
it
Frequency Division Multiple Access
frequency

9.
• Time is divided into slots and only one mobile terminal transmits during
each slot
– Like during the lecture, only one can talk, but others may take the floor in
turn
• Each user is given a specific slot. No competition in cellular network
– Unlike Carrier Sensing Multiple Access (CSMA) in WiFi
Time Division Multiple Access
Guard time – signal transmitted by mobile
terminals at different locations do no arrive
at the base station at the same time

10.
∗ Use of orthogonal codes to separate different
transmissions
∗ Each symbol of bit is transmitted as a larger number
of bits using the user specific code – Spreading
∗ Bandwidth occupied by the signal is much larger than the
information transmission rate
∗ But all users use the same frequency band together
Code Division Multiple Access
Orthogonal among users

12.
∗ Abbreviation for Global System for Mobile
Communications
∗ Concurrent development in USA and Europe in the
1980’s
∗ The European system was called GSM and deployed in
the early 1990’s
GSM

13.
∗ Voice, 3.1 kHz
∗ Short Message Service (SMS)
∗ 1985 GSM standard that allows messages of at most 160
chars. (incl. spaces) to be sent between handsets and
other stations
∗ Over 2.4 billion people use it; multi-billion $ industry
∗ General Packet Radio Service (GPRS)
∗ GSM upgrade that provides IP-based packet data
transmission up to 114 kbps
∗ Users can “simultaneously” make calls and send data
∗ GPRS provides “always on” Internet access and the
Multimedia Messaging Service (MMS) whereby users can
send rich text, audio, video messages to each other
∗ Performance degrades as number of users increase
∗ GPRS is an example of 2.5G telephony – 2G service similar
to 3G
GSM Services

14.
∗ Physical Channel: Each time slot on a carrier is referred to as a
physical channel
∗ Logical Channel: Variety of information is transmitted between
the MS and BTS.
Different types of logical channels:
1.Traffic channel
2.Control Channel
GSM Channels
Downlink
Uplink
Channels

18.
∗ A small smart card
∗ Encryption codes needed to identify the subscriber
∗ Subscriber IMEI number
∗ Subscriber’s own information (telephone directory)
∗ Third party applications (banking etc.)
∗ Can also be used in other systems besides GSM, e.g.,
some WLAN access points accept SIM based user
authentication
Subscriber Identity Module

19.
• Transcoding Rate and Adaptation Unit (TRAU)
– Performs coding between the 64kbps PCM coding used
in the backbone network and the 13 kbps coding used
for the Mobile Station (MS)
• Base Station Controller (BSC)
– Controls the channel (time slot) allocation implemented
by the BTSes
– Manages the handovers within BSS area
– Knows which mobile stations are within the cell and
informs the MSC/VLR about this
• Base Transceiver System (BTS)
– Controls several transmitters
– Each transmitter has 8 time slots, some used for
signaling, on a specific frequency
Base Station Subsystem

21.
∗ One database per operator
∗ Contains all the permanent subscriber information
∗ MSISDN (Mobile Subscriber ISDN number) is the
telephone number of the subscriber
∗ International Mobile Subscriber Identity (IMSI) is a 15
digit code used to identify the subscriber
It incorporates a country code and operator code
∗ IMSI code is used to link the MSISDN number to the
subscriber’s SIM (Subscriber Identity Module)
∗Charging information
∗Services available to the customer
∗ Also the subscriber’s present Location Area Code, which
refers to the MSC, which can connect to the MS.
Home Location Register

23.
∗ The cells overlap and usually a mobile station can ‘see’
several transceivers (BTS’s)
∗ The MS monitors the identifier for the BSC controlling
the cells
∗ When the mobile station reaches a new BSC’s area, it
requests an location update
∗ The update is forwarded to the MSC, entered into the
VLR, the old BSC is notified and an acknowledgement
is passed back
Location Updates

24.
∗ When a call is in process, the changes in
location need special processing
∗ Within a BSS, the BSC, which knows the
current radio link configuration (including
feedbacks from the MS), prepares an available
channel in the new BTS
∗ The MS is told to switch over to the new BTS
∗ This is called a hard handoff
∗ In a soft handoff, the MS is connected to
two BTS’s simultaneously
Handoff (Handover)

25.
When a MS enters another operators network,
it can be allowed to use the services of this
operator
∗ Operator to operator agreements and contracts
∗ Higher billing
The MS is identified by the information in the
SIM card and the identification request is
forwarded to the home operator
∗ The home HLR is updated to reflect the MS’s
current location
Roaming

30.
GSM Evolution to 3G
GSM
9.6kbps (one timeslot)
GSM Data
Also called CSD
GSM
General Packet Radio Services
Data rates up to ~ 115 kbps
Max: 8 timeslots used as any one time
Packet switched; resources not tied up all the time
Contention based. Efficient, but variable delays
GSM / GPRS core network re-used by WCDMA (3G)
GPRS
HSCSD
High Speed Circuit Switched Data
Dedicate up to 4 timeslots for data connection ~ 50 kbps
Good for real-time applications c.w. GPRS
Inefficient -> ties up resources, even when nothing sent
Not as popular as GPRS (many skipping HSCSD)
EDGE
Enhanced Data Rates for Global
Evolution
Uses 8PSK modulation
3x improvement in data rate on short distances
Can fall back to GMSK for greater distances
Combine with GPRS (EGPRS) ~ 384 kbps
Can also be combined with HSCSD
WCDMA

35.
∗ Wide band CDMA technology is selected for UTRAN air
interface
∗ WCDMA
∗ TD-SCDMA
∗ Base stations are referred to as Node-B and control
equipment for Node-B is called as Radio Network
Controller (RNC).
∗ Functions of Node-B are
∗ Air Interface Tx/Rx
∗ Modulation/Demodulation
∗ Functions of RNC are:
∗ Radio Resource Control
∗ Channel Allocation
∗ Power Control Settings
∗ Handover Control
∗ Ciphering
∗ Segmentation and reassembly
UTRAN

36.
High Speed Packet Access (HSPA) is an amalgamation of two
mobile telephony protocols, High Speed Downlink Packet
Access (HSDPA) and High Speed Uplink Packet Access
(HSUPA), that extends and improves the performance of
existing WCDMA protocols
3.5G introduces many new features that will enhance the
UMTS technology in future. 1xEV-DV already supports most of
the features that will be provided in 3.5G. These include:
- Adaptive Modulation and Coding
- Fast Scheduling
- Backward compatibility with 3G
- Enhanced Air Interface
3.5G (HSPA)